1. Field of the Invention
This disclosure relates to the field of detection of rotational motion. Specifically, to rotational seismometers and methods for using and calibrating them.
2. Description of Related Art
The prompt detection of earthquakes and other seismic events has gained interest in recent years. As the world has become more populated, the possibility of seismic events effecting population centers has become greater. In order to both detect seismic events, and to study seismic events for future preparation, it is necessary to use appropriate instrumentation. Originally, it was believed that the motion of an earthquake was primarily translational. However, recent learning, and an improvement in available instrumentation, has shown that rotational movement components, particularly near the epicenter of a seismic event, may actually be more important.
In part because of these realizations, the last years have witnessed revolutionary changes in rotational seismology resulting from the combinations of greatly enhanced capabilities of geophysical instrumentation and the appearance of first commercially available rotational seismometers. Such sensors could be employed in areas of high seismicity, where the translational and rotational motions have comparable orders of magnitude. This is especially true for the near zones of strong shallow earthquakes. The measurement of this frequently observed rotational motion in the vicinity of the epicenters of strong earthquakes can be extremely valuable in earthquake engineering, since buildings and other structures have been discovered to generally be quite vulnerable to the torsional stresses created by rotational motions.
A variety of angular sensors are commercially available to detect rotational motion. Some of these feature quite excellent resolution, and offer a frequency band extending into the dc range. Rather than being true rotational seismometers, however, such devices are actually very low frequency accelerometers that measure the tilt of their foundation relative to the local gravity vector. Since gravity is indistinguishable from any other inertial acceleration, these instruments are inherently incapable of separating pure rotation from horizontal accelerations.
A natural method of measuring “pure rotations” would be to use two identical vertical seismometers placed a certain distance from each other, so that the rotational motion can be derived from the difference between the two outputs. Interestingly enough, the concept for a pendulum-based rotational seismometer and its use to correct horizontal seismic signals were put forward a century ago by the Prince Boris B. Golitsyn.
However, starting with Golitsyn's early experiments, and in many subsequent attempts, the resolutions attained by the proposed methods were very poor, since even the smallest differences between the two instruments can lead to large errors. Indeed, it was shown that in order to achieve a tilt measurement accuracy of even 10−7 rad, the maximum acceptable difference between the two seismometer's characteristics must be about 10−4%, a consistency which is practically impossible to realize.
There are also a few “true” rotational sensors that are currently known. That is, sensors which measure angular motion and are generally insensitive to translational accelerations. The best known and most accurate types are: Magnetohydrodynamic Angular Rate Sensors where the typical passband is 0.5-1000 Hz. Further, angular resolution at the low cutoff frequency is ˜1.6.10−7 rad. As it is unlikely that this device's passband can be extended even to a period of 100 sec, these types of devices have generally been considered unusable for seismic use.
Alternatively, MEMS-based gyroscopes and fiber optic rate gyroscopes put out a signal proportional to the angular velocity in the 0 to 100 Hz band, with a resolution of about 10−5 rad/sec. The instrument's sensitivity to translational acceleration is specified as 10−4 rad/sec/g, which is orders of magnitude less than the desired value for seismic applications. Further, MEMS-based systems, in particular, provide limited short term stability (0.05°/sec over 100 sec at constant temperature) and long term stability (1°/sec over 1 year) which are inadequate for seismic applications.